This invention relates to dispersing agents, and to coal slurries containing such dispersing agents. More particularly, the invention relates to dispersing agents comprising copolymers of acrylonitrile or methacrylonitrile with acrylic acid or methacrylic acid, or salts of these acids.
In recent years there has been increasing interest in the manufacture of high solids coal/water slurries which have good stability during transit and storage, but which can be readily burned as a substitute for heavy fuel oil in various applications. The coal should be as fine as possible, so that it burns rapidly and completely, and the slurry must be sufficiently fluid to flow under gravity or when pumped. It should not deposit sediment, nor should clear water separate at the top surface, when the slurry is stored for any length of time.
In recent years, there has been increasing interest in the long distance transport of coal by pumping a slurry of coal particles in water through a pipeline. The first Patent for the transport of coal in water dates back to 1891, and a short pipeline was operated as long ago as 1914. Since the 1950's several pipelines having lengths exceeding 150 kilometers have been constructed and are in operation.
To ensure ease of transport, it has been necessary to pay careful attention to the particle size distribution of the solids, and to employ various dispersing agents to ensure that the particles remain distributed in the aqueous phase. Some quite complex grinding and grading procedures have been suggested (see WO No. 83/04046), but there are practical and economic difficulties in applying such techniques upon a large scale.
A number of different types of dispersing agent have been proposed. One type comprises half ethers or half esters or polyoxyalkylene glycols, either along (WO No. 83/04044) or U.S. Pat. No. 4,358,293) or in association with ionic surface active agents, such as polyoxyalkylene derivatives of sulfosubstituted or quaternary ammonium-substituted phenols (WO No. 83/04045).
Another approach has been to make the surface of coal particles mutually repulsive by the addition of Zwitterionic amphiphiles, such as lecithin or alkyl betains (see WO Nos. 83/03617 and 03618). These materials can also be associated with polyalkylene oxides (see WO No. 83/00500).
Various types of sulphonates have been employed, including lignin and humic (EP-A-0089766, EP-A-0092353, EP-A-0050412, EP-A-0065259, U.S. Pat. No. 3,019,059, U.S. Pat. No. 4,330,301.
The use of polyacrylates has been disclosed in U.S. Pat. No. 4,217,109 and WO No. 83/00501 and acrylate/acrylamide copolymers have been disclosed in U.S. Pat. No. 4,370,429.
There is still, however, a need for effective dispersing agents, since in practice it has not been found possible to manufacture and store coal slurries having concentrations much exceeding 45 to 55% by weight, without the addition of a dispersant. Such slurries would not be suitable as fuels, without needing further treatment, since they comprise too much water and not enough coal.
In U.S. Pat. No. 3,524,682 there is described a polyelectrolyte for use in the pumping of coal slurries. This polyelectrolyte may comprise polymers of acrylonitrile or acrylic acid or copolymers thereof: optionally with small amounts of further monomers. The said polyelectrolyte must have a molecular weight of at least 100,000. When this polyelectrolyte is added to coal slurries it forms a suspension with a solids content of up to 60% of coal particles.
The coal slurries formed in this way become free-flowing only when pumped at high pressure and at high velocities. The slurry reverts to a suspension when static.
Such suspensions are of little use where gravity feed of a slurry which has a high solid content of coal is required, such as in coal-fired power generating plant.